Commercially available hydropneumatic axle suspensions have a level control. When the axle load increases, the vehicle body sinks due to the changing pressures in the respective suspension cylinder. A level control valve is then actuated until the piston sides of the suspension cylinders as hydropneumatic actuators have reached the original starting level again. As the axle load decreases, the body can be raised accordingly. The level control valve connects the piston chambers of the suspension cylinders in use at the time to the tank line of the fluid system until the original level is restored.
In a development of this idea, DE 102 32 769 A1 discloses a hydropneumatic axle suspension for vehicles with dramatically changing axle loads, in particular for front axles on truck tractors with hydraulic suspension cylinders connected to hydropneumatic accumulators, a suspension circuit of the piston chambers pressure-controlled by a level control and a pressure-controlled suspension circuit of the annuli. A load-sensing pump can be set via a control line (load sensing line) by a control pressure for generating pressure. A pressure control valve keeps the pressure level constant in the suspension circuit of the annuli. In this known solution, the pressure control valve is a proportionally controlled valve triggered by a control current from an electrical control device as control electronics to proportionally control the axle spring rate as a function of at least one sensor signal between the constant annulus pressure level in partial regions and, if necessary, to override a selectable constant pressure to represent a given axle spring rate.
In the illustrated known solution, the annulus pressure is proportionally controlled in partial regions between the constant annulus pressure levels for more rapid pressure matching of the suspension accumulator. A proportional valve is hydraulically connected such that there is load sensing control with pressure relief for a control pump. In the level position, the two suspension circuits of the piston and ring sides in the known solution are kept at the set pressure level free of leaks by two pressure-tight 2/2-way valves. In the level control process, those valves are switched accordingly so that the special resettable hydraulic check valves, which were necessary in the past, can be omitted. In addition to the automatic changing of the spring rate by automatic annulus pressure control, it is possible in the known solution to intentionally change the pressure level in the annulus as necessary by external intervention to be able to match the spring rate to the given operating conditions. To be able to transmit the current pressure values in the piston chambers of the two suspension cylinders to the control electronics, a pressure sensor is connected to them. The sensor is protected against overloading by the pressure limiting valve of the fluid system.
With the known solution, an inherently economical solution of small geometry is devised for the control block of an axle suspension enabling a high level of comfort and inputs with respect to the individual spring rate which is to be set. In spite of these advantages, the known solution, however, for triggering the system requires a proportional pressure control valve and a shuttle valve which must transmit the higher pressure prevailing in one fluid branch to the control line (load-sensing line) for triggering a load-sensing pump. In addition to the two 2/2-way valves, additional valve components are required. These additional valve components are expensive and can also endanger the operating reliability of the axle spring control by their failure.